This invention pertains to the art of measurement of torque applied as a series of sharp impulses by an impact wrench or the like.
The invention is particularly applicable to the calibration of hand held impact wrenches and the like.
The fastening together of parts is one of the most common of manufacturing operations. One often used means of fastening parts together is the use of threaded fasteners. Nuts and bolts, nuts and threaded studs, and other threaded fasteners are used in a wide variety of manufacturing operations. Many different types of threaded fasteners and tools for applying threaded fasteners have been developed to meet the needs of manufacturers in these operations.
For threaded fasteners to reliably perform in holding parts together, tension must be developed in the body and thread length of a bolt, stud or the like. This tension holds opposing surfaces of the threaded nut and bolt tightly together and, in many cases, slightly deforms the threads. High frictional forces are maintained preventing the nut and bolt from unscrewing from one another.
Tension is normally applied to the body and thread of a bolt by rotating the nut with respect to the bolt and continuing tightening rotation until appropriate bolt tension is achieved. However, in most manufacturing operations this bolt tension cannot be directly measured or controlled. Instead, the torque, that is, the rotational force causing the bolt or nut to rotate with respect to the other, is often measured or controlled.
Hand held, unpowered torque wrenches or continuously acting powered torque wrenches are often used to apply the tightening torque. These tools can be calibrated on existing calibration units without extreme difficulty. Therefore, a uniform, known torque can be applied to fasteners by using calibrated wrenches. Application of such a uniform known torque to fasteners is essential in manufacturing operations to be certain that fasteners will remain fastened.
Many manufacturing and other industrial organizations use powered impact wrenches to apply nuts to bolts and the like.
Powered impact wrenches apply torque as a series of sharp rotational blows. Short, sharp periods of torque having a high instantaneous value are applied to the nut ot bolt through a rigid driving head. Measuring and controlling the torque applied to a fastener by an impact wrench has been difficult in the past.
Past methods and apparatus for testing and calibrating impact wrenches have inherent inaccuracies of several kinds. Some methods use a mechanical portion having two parts such as a nut and a stud which move with respect to one another. Torque is applied to the nut which rotates on the stud. The tension imposed on the stud is then measured. Errors are introduced as torque is not directly measured. The state of lubrication of the mating threads on the test nut and bolt will affect the tension produced on the bolt by a given torque and therefore cause changes in readings from on test to another. Wear of the mating threaded surfaces will also cause inaccurate readings. Changes in lubrication, wear and other factors affecting the friction between the mating threads will cause errors. The same torque applied to the same calibrator at different times will result in a different reading. Because of this, exact calibration of systems cannot be held for appreciable periods of time.
Another existing means of measuring torque is by the use of strain gauges on a small body to which the torque is applied. As the torque is applied, the body undergoes torsion and the resistance of the strain gauges changes in accordance with this torsion providing a signal representative of the torque applied. This is acceptable for wrenches and devices applying a constant steady torque or a slowly varying torque. However, such systems do not accurately reflect the true nature of the impulse applied by an impact wrench. When used with an impact wrench, the meter usually measures displacement caused directly by the torque exerted by the impact wrench which is dependent upon the mass, moment of inertia and coupling of the entire system. Errors are introduced by inconsistent coupling and resonances within the mechanical system. Moreover, the mass of the total system subject to torque is sometimes significantly different for different wrenches resulting in inaccuracies.
Because of the inherent inaccuracies in strain gauge type instruments when used with impact wrenches, use of the instruments with impact wrenches was discouraged.
It has also been found that the hammer-like mechanism which produces pulses of torque in typical impact wrenches sometimes causes a rebound torque in the driving portion of the wrench and the elements driven by the wrench. Such rebounds are seen as torque applied in the direction opposite of desired torque. The rebound torque produces an erratic spurious signal which interferes with torque measurement in rigidly coupled systems.
The present invention eliminates all of the lubrication, wear and frictional problems of torque measuring devices having movable interacting parts and overcomes the inherent inaccuracies of the prior art by use of a unitary mechanical structure not subject to frictional inconsistencies or unwanted resonances.
In accordance with the present invention, a torque wrench calibrator is provided comprising a beam having a stationary end plate, a rod portion, a free end portion, and an adapter fixed to the free end portion to receive the driving portion of an impact wrench. At least one strain gauge is fixed to the rod portion of the beam oriented to respond to angular displacement of the rod portion by changing its electrical resistance. The maximum change in the resistance of the strain gauge is read by an electronic peak detector. Importantly, the mass and cross-section of the beam, when coupled with the driving portion of an impact wrench, has an effective moment of inertia such that its natural period of oscillation is significantly greater than the duration of torque impulses applied by the wrench. The maximum displacement of this oscillation is directly proportional to the torque applied and is read by the peak detector.
Further in accordance with the invention, the beam free end portion is comprised of two rigidly connected pieces having different moduli of elasticity, and different damping characteristics, such as steel and aluminum, such that unwanted rebounding is eliminated.
Yet further in accordance with the invention, an electronic reading circuit is provided comprising an amplifier receiving an input from the strain gauges disposed upon the beam and having an output signal proportional to the resistance of the strain gauges; a peak detector recognizing the highest value of the output signal of the amplifier and holding this value; and, a display means displaying this value which is proportional to the value of the impulse of torque applied by the power tool being calibrated.
Yet further in accordance with the invention, a zeroing and compensation circuit is provided which will return the peak detector to its initial condition and compensate for any drift or inaccuracies in the circuit thereby automatically providing an accurate zero starting point for measuring.
Still further in accordance with the invention, a pulse counting circuit is provided which will count the number of pulses per minute applied to the calibrator by an impact wrench and display this number.
The pulse counting circuit includes means for generating a reference signal equal to a fraction of the value held in the torque measuring peak detector and means comparing this reference signal to the instantaneous torque signal, thereby creating an output consisting of a pulse for each pulse of torque. Because two signals based on the value of torque applied are compared, the counter accurately detects and counts impulses over a wide range of torque values.
Yet further in accordance with the invention, a method of calibrating impact wrenches is provided in which a torque is applied as an impulse or a series of impulses to a unitary beam having a long period of natural oscillation such that the beam responds to short periods of torque by oscillating at its natural frequency; measuring the maximum angular displacement caused by the natural oscillation of the beam and displaying a number proportional to this maximum displacement.
The principal object of the invention is the provision of a new and improved impact wrench calibrating device which accurately measures the value of impulses of torque applied.
Another object of the invention is the elimination of frictional error elements in the measurement of torque applied by an impact wrench.
Another object of the invention is the elimination of rebound torque from an impact wrench torque measuring apparatus.
Yet another object of the invention is the provision of an impact wrench calibrator which can measure the value of an impulse applied by an impact wrench, the number of impulses per minute applied by an impact wrench, and the instantaneous torque applied by a continuous type of torque wrench all in one compact instrument.
Still another object of the invention is the provision of an impulse counter which will accurately count impulses over a wide range of impulse value or strength.
Yet another object of the present invention is the provision of an impact wrench calibrator which accurately measures torque applied by various wrenches having different driving portion mass and/or geometry.